Caratterizzazione e differenziazione mineraiogica dei livelli sabbiosi tardo-quaternari del litorale veneziano

Abstract

This parer reports the results of a mineralogical investigation on 170 sand samples from 18 boreholes, 25-30 m deep and representative of Upper Quaternary sand sediments underlying the sandy littoral bar that separates the Lagoon of Venice from the open Adriatic Sea. A minimum of 200 grains, per sample, randomly chosen, have been analysed using an Electron Microprobe (EPMA) equipped with an Energy Dispersion Spectrometer (EDS). The statistical elaboration of the analytical data indicates that the percentages of quartz, feldspars, carbonates, mica and chlorites for the sediments of the continental depositional environments are quite different from those of marsne-lagoonai environments. The mineralogical differentiation of sand sediments and their detailed characterisation, can be used to more accurately define the Holocene/Pleistocene boundary, particularly in those areas of the lagoon where the limit is not represented by the overconsolidated clay layer, typical of the last Pleistocene continental sedimentation, locally known as caranto. The average mineralogical composition also shows vertical and lateral variations. Generally, vertical variations are linked to changes in depositional environment conditions mainly due to climatic changes during the past 25,000 years. The lateral variations between the northern and southern areas of the Venetian littoral are related to the petrology of the source areas of sediments, the carbonate-rich Tagliamento, Piave, Sile, Adige and Brenta Rivers basins and the silicate-rich Po River basin from north to south, respectively. This study highlights that the average mineralogical composition of sands deposited during the last stage of the WQrmian glaciation is relatively homogeneous while sand deposited during the post-glacial Flandrian transgression, shows very distinct variations.